al/bf-mie-fix

[Pwhsky]

Adds a line to convert a list to an xp-array when resolving fields for Mie scatterers.

line 2002 in optics.py:

...
     field_arrays.append(arr)

field = xp.sum(field_arrays, axis=0)

Will fail when attempting xp.sum due to field_arrays being a list.

Adding an intermediate conversion:

...
     field_arrays.append(arr)

field_arrays = xp.asarray(field_arrays)
field = xp.sum(field_arrays, axis=0)

Resolves this.

[edudc]

xp.asarray(field_arrays) fixes the NumPy case, but still fails for torch when
field_arrays is a list of multi-element tensors:

ValueError: only one element tensors can be converted to Python scalars

The numpy and torch safe operation is stack instead of xp.asarray, then sum:
field = xp.sum(xp.stack(field_arrays, axis=0), axis=0)
This also preserves torch autograd through the stacked scattered fields.

[cmanzo]

Please implement Eduard suggestions